A conventional hydraulic system includes a pump that draws low-pressure fluid from a tank, pressurizes the fluid, and makes the pressurized fluid available to multiple different actuators for use in moving the actuators. In this arrangement, a speed of each actuator can be independently controlled by selectively throttling (i.e., restricting) a flow of the pressurized fluid from the pump into each actuator. For example, to move a particular actuator at a high speed, the flow of fluid from the pump into the actuator is restricted by only a small amount. In contrast, to move the same or another actuator at a low speed, the restriction placed on the flow of fluid is increased. Although adequate for many applications, the use of fluid restriction to control actuator speed can result in flow losses that reduce an overall efficiency of a hydraulic system.
An alternative type of hydraulic system is known as a meterless hydraulic system. A meterless hydraulic system generally includes a pump connected in closed-loop fashion to a single actuator or to a pair of actuators operating in tandem. During operation, the pump draws fluid from one chamber of the actuator(s) and discharges pressurized fluid to an opposing chamber of the same actuator(s). To move the actuator(s) at a higher speed, the pump discharges fluid at a faster rate. To move the actuator with a lower speed, the pump discharges the fluid at a slower rate. A meterless hydraulic system is generally more efficient than a conventional hydraulic system because the speed of the actuator(s) is controlled through pump operation as opposed to fluid restriction. That is, the pump is controlled to only discharge as much fluid as is necessary to move the actuator(s) at a desired speed, and no throttling of a fluid flow is required.
An exemplary meterless hydraulic system is disclosed in U.S. Pat. No. 4,369,625 to Izumi et al. (“the '625 patent”). The '625 patent describes a multi-actuator meterless hydraulic system having flow combining functionality. The hydraulic system of the '625 patent includes a swing circuit, a boom circuit, a stick circuit, a bucket circuit, a left travel circuit, and a right travel circuit. Each of the swing, boom, stick, and bucket circuits have a pump connected to a specialized actuator in a closed-loop manner. In addition, a first combining valve is connected between the swing and stick circuits, a second combining valve is connected between the stick and boom circuits, and a third combining valve is connected between the bucket and boom circuits. The left and right travel circuits are connected in parallel to the pumps of the bucket and boom circuits, respectively. In this configuration, any one actuator can receive pressurized fluid from more than one pump.
Although an improvement over existing meterless hydraulic systems, the functionality of the meterless hydraulic system disclosed in the '625 patent is limited. In particular, none of the individual circuit pumps are capable of providing fluid to more than one actuator simultaneously. Thus, operation of connected circuits of the system may only be sequentially performed. For example, when the stick is operating in a high load condition, the first combining valve may temporarily combine fluid provided to the stick by the stick circuit with supplemental fluid from the swing circuit. While such a combined flow may assist in meeting stick demand, the system is not capable of operating both the stick circuit and the swing circuit simultaneously while providing the combined flow to the stick. As a result, operation of the hydraulic system disclosed in the '625 patent may be limited in certain situations.
In addition, the speeds and forces of the various actuators may be difficult to control. For example, the hydraulic system of the '625 patent employs fixed displacement motors in the left and right travel circuits, as well as the swing circuit. These motors are only capable of operating at speeds and rotation directions determined by the corresponding pumps of the bucket, boom, and swing circuits, respectively. Such a configuration does not allow for independent speed and/or rotation direction control of the actuators unless the displacement and/or rotation direction of the associated pumps is also changed. Controlling the actuators in this way may be difficult and/or undesirable in certain applications.
The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth above and/or other problems of the prior art.